The page says the timing gap bites hardest on the smallest pack. The guarantee, though, is 60 days on every pack.
In days, the gap is identical: the refund window closes at day 60 and the brand's suggested timeline starts at 90, whatever the pack. What differs is what you hold at day 60. On the 2-bottle pack you will have used nearly all the product. On the 3-bottle pack you still hold one bottle, and on the 6-bottle pack four.
So the smaller pack cannot reach 90 days at all without buying more, while the larger packs can reach it but only after the guarantee ends.
The smaller pack's problem is not enough product to follow the brand's timeline. The larger packs' problem is money spent beyond the refund window. Neither is better; they are different risks, and the right one depends on how sure you are.